Whale, Dolphin Deaths in Gulf Spill Underestimated

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The death toll of animals that perished as a result of the
Deepwater Horizon/BP oil spill in the Gulf of Mexico may be 50
times higher than presently believed, according to a new study in
the latest issue of Conservation Letters.

Until now, fatality figures have primarily been based on the
number of recovered carcasses. Data on this varies depending on
the source and the date of the count, but the authors report that
as of Nov. 7, 2010, 101 whale, dolphin, and porpoise carcasses
had been detected across the Northern Gulf of Mexico.

Past numbers of carcasses reflect just 2 percent of actual animal
deaths, according to the study, so the true number of fatalities
for cetaceans alone as a result of the spill could be in the
thousands.

“Our calculations are rough, but are a good starting point, and
are far better than assuming that the bodies on the beach
represent the sum total of the damage,” lead author Rob Williams
told Discovery News.

Williams, a member of the University of British Columbia’s Marine
Mammal Research Unit, and colleagues focused their analysis on 14
cetaceans from the Gulf region. These included sperm whales,
killer whales, Atlantic spotted dolphins, spinner dolphins, and
others.

The 2 percent figure resulted from the team multiplying recent
species abundance estimates for these marine mammals by the
species mortality rate, and then dividing the mean number of
documented strandings each year by the estimate of annual
mortality.

Very few remains are found because of decomposition, bodies lost
at sea, predators consuming dead individuals, and for other
reasons, according to the scientists. Since distance from shore
and water depth can also impede finding remains, most Deepwater
disaster victims may never be found. The oil spill took place 40
miles offshore and in close to 5,000 feet of water.

Despite disturbing images showing animals, like birds, dying
under thick coatings of oil after the spill, there is still no
evidence proving that cetaceans died as a direct result of the
April 2010 Deepwater explosion and spill.

“Like many others we are waiting for the necropsies to be
complete, and the results peer reviewed and reported,” Williams
explained. “At that point, we can start a discussion about
(precisely) how many animals in total were affected by the spill,
and how large these mortalities were relative to population
size.”

He and his colleagues believe that their calculations can be used
to estimate marine mammal deaths due to other human-related
causes, such as ship strikes and interactions with fishing gear.
They might also be applied to past disasters, such as the Exxon
Valdez oil spill.

Craig Matkin, director of the North Gulf Oceanic Society, has
extensively studied marine mammals affected by that 1989 spill.
He believes the new study is “welcome and important.”

Matkin told Discovery News that relatively few cetacean carcasses
were found after the Exxon Valdez event. He said he and his team
“would not have known that we lost 13 killer whales from the
major resident pod in Prince William Sound if we were not able to
determine the deaths because of individual tracking of each whale
through photo identification.”

He added, that most whales and dolphins sink after dying, and
don't end up stranded at shore.